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Finding a direct link from sodium to heart disease

I like to tell my medical students that "medicine is not math." Remember that Transitive Property of Equality you might have learned about in Algebra? Where if A = B and B = C, then A always equals C?

Medicine is not like that. (Would that it were!) In medicine, sometimes A = B and B = C, but A has no relation to or effect on C.

Sodium and heart disease is a good example: we know that sodium intake is related to blood pressure (this is our A = B), and that blood pressure is related to heart disease (B = C), but the evidence linking sodium intake directly to heart disease (A = C) has been rather limited, as the Institute of Medicine noted not long ago.

Recently there have been studies published that have suggested that lower levels of sodium intake actually increase one's risk of dying from cardiovascular disease. Other studies showed an increase in risk of developing cardiovascular disease at the highest and lowest levels of sodium intake. Since these studies contradict current public health efforts to reduce people's sodium intake, naturally the media jumped on them. You've probably heard something about all this. The problem, however, is that those studies were not well designed: problems ranged from participant selection (people with existing illnesses, for example) to inexact measurements of participants' sodium intake (some of which were estimates based on questionnaires about people's diet).

Fortunately a team of researchers from Harvard, Johns Hopkins, and Tulane University (Circulation DOI:10.1161/CIRCULATIONAHA.113.006032) examined data gathered through two long-term studies of otherwise healthy, overweight or obese persons with high normal blood pressure, which focused on preventing high blood pressure through various means, including weight loss and actively reducing sodium intake (but not by taking blood pressure medications). These studies used 24-hour urine collections, performed on multiple occasions throughout the studies, to accurately measure the exact amount of sodium in the individual participant's diet. (These 24-hour urine collections are considered the gold standard for assessing sodium intake.)

The researchers excluded those who were specifically assigned to sodium reduction and averaged the sodium intakes tested via urine collection for the remaining ~3,000 individuals. They then grouped the participants into four increasing levels of dietary sodium: <2,300 milligrams per day, 2,300 to <3,600 per day, 3,600 to <4,800, and over 4,800 milligrams per day.

After taking into account such variables as Body Mass Index, exercise, or smoking levels (including whether any of those changed), ethnicity, and family history of heart disease, they discovered that compared to those who were consuming 3,600 to 4,800 milligrams of sodium per day, those who kept their sodium intake under 2,300 milligrams (the current recommendation for the majority of people) were 32% less likely to experience a cardiovascular event such as heart attack or stroke. In fact, every additional 1,000 milligrams of sodium over 2,300 milligrams increased an individual's risk by 17%.